Lecture 9 Genome Instability and Checkpoints (WIP) Flashcards
What is meant by genome instability
An elevated rate of genetic alterations
Tumorigenesis is a multi-stage process T or F
T
Give some examples of checkpoints throughout the cell cycle
Restriction point that governs entry into S phase from G1 intra-S-phase checkpoint that checks for DNA damage or stalked replication forks G2M phase checkpoint that checks that all DNA has been replicated and then the metaphase to anaphase transition in mitosis that checks whether all chromosomes are attached to the spindle
Why is the intestinal epithelium used as a model for the changes in cancer cells over time
This is the best characterised and understood cancer in terms of the chromosomal abnormalities that correspond with its progression
Outline the progression of intestinal epithelium towards tumorigenesis
Normal epithelium hyperplastic epithelium early/intermediate/late adenoma carcinoma metastasis
Below is a diagram outlining the progression of intestinal epithelium during tumorigenesis. Fill in above the arrows with the specific chromosomal abnormality that causes each transition
Loss of APC DNA hypomethylation activation of KRas loss of Smad4 loss of p53
Give examples of some of the chromosomal changes that occur in the progression of cells during tumorigenesis
Loss of tumour suppressor genes proto-oncogene to oncogene transitions and changes in epigenetic regulation of the genome
What is meant by a driver mutation
A mutation that emerges early in cancer and occurs in a large majority of all cancers of a certain type
There are over 15 different driver mutations in colorectal cancer give some examples of these
APC PIK3CA KRas TP53 FBXW7 β-catenin axin-2
Why are driver mutations better therapeutic targets in the development of cancer treatments
Because mutations in these genes are extremely common and occur early so this will maximise the patients who could benefit and would mean that the cancer is stopped early in its progression
What is significant about the various different mutations that occur at each point in tumorigenesis
Mutations at each stage in tumorigenesis are often components of the same pathway
What hypothesis was proposed to explain how tumour cells manage to accumulate mutations
The mutator hypothesis was proposed to explain the accumulation of all the mutations in tumour cells. This is the idea that tumour cells acquire initial mutator gene mutations that then increases the rate of subsequent mutations
What are the two type of instability seen in cancer cells and how do they differ
Chromosomal instability – chromosomal rearrangements and aberrations as well as loss of heterozygosity aneuploidy/polyploidy and gene amplifications. Microsatellite instability – point mutations and base substitutions as well as microdeletions or insertions giving rise to missense mutations
Microsatellite and chromosomal instabilities are common in normal cells T or F
F – they are extremely rare occurring once in every 10million cell divisions
Where is it that mutator genes are often found
Form part of the DNA repair systems
What are the different methods of DNA repair
Base excision repair nucleotide excision repair homologous recombination and non-homologous end joining
What mechanisms exist to prevent DNA replication and processing errors
Precursor control polymerase proof-reading and mismatch repair
What is meant by integral dependency. How does this relate to the cell cycle
Integral dependency is where one component is reliant on the previous component. It suggests that in order for the cell cycle to progress each phase needs to be completed before the next one can begin
Give an example of integral dependency
Bacteriophage assembly – whereby one component of virus assembly depends on the previous component. In other words the way in which the virus assembles is inherent to the structure of the virus
What is the effect of RAD9 mutations in the growth of S. cerevisiae
Cells lacking RAD9 are healthy in the absence of extrinsic interference. Hence the growth curves of RAD9 and rad9 yeast cells are the same
What were the results of experiments carried out with yeast harbouring temperature-sensitive cdc9 mutations
FACS analysis of cdc9 mutants at permissive temperature looks normal however FACS analysis of cdc9 mutants at a restrictive temperature indicates G2 arrest
What is the role of cdc9 and how does this account for what is seen in the temperature-sensitive mutants
Cdc9 codes for a DNA ligase that is important in lagging strand synthesis where it is required to join Okazaki fragments during DNA replication. If you lack cdc9 there will be complete DNA synthesis but this DNA will be fragmented. Hence cdc9 mutants at restrictive temperatures arrest after S phase as all of the DNA is synthesised but is fragmented so that M phase cannot proceed
What is meant by cell viability how does it differ from cell cycle arrest
Viability is the ability of cells to grow and make daughter cells. It isn’t related to cell cycle arrest as in the case of temperature-sensitive mutants changing the permissive temperature of a cdc mutant back to a permissive one will result in the cells coming out of arrest and therefore being viable
Describe what these results obtained by Lee Hartwell show about the viability of cdc9/rad9 mutants
These graphs show that wild type yeast as well as cdc9 and rad9 mutants are all viable a hence have a near 100% viability. However the rad9/cdc9 combination mutants have drastically reduced viability. Interestingly the longer you leave the rad9/cdc9 cells the more rapidly that viability is reduced. This implies that cells undergoing replication stress such as due to non-functional DNA ligase/cdc9 require RAD9 in order to maintain their viability. Once this is also lost cells are no longer viable
